Vertical kiln control

ABSTRACT

Uniform heat treatment of particulate material of a nonuniform gradation in a vertical kiln or retort is effected by regulating either the flow of particulate material from the outlet of the kiln or the heat input to the kiln in response to differential pressure changes between fluid, such as combustion-supporting fluid and process gas flowing into and from the burning or calcining zone, to result in a balance between particulate mass flow and heat input, and a substantially uniformly heat-treated product.

United States Patent 1191 Summer Nov. 19, 1974 VERTICAL KILN CONTROL 2,814,479 11/1957 Leone 432 36 [75] Inventor: James R. Summer, Blum Hill, Tex.

0 Primary Examiner-John J. Camby Asslgneei Round Rock Llme p y Blum, Attorney, Agent, or Firm-Richards, Harris &

Medlock [22] Filed: July 30, 1973 211 App]. No.: 383,484 [57] ABSTRACT Uniform heat treatment of particulate material of a nonuniform gradation in a vertical kiln or retort is efgl iiiiiiiiiiiiiiiiiiiiiii 432/47 432/ fected by regulating either the flow of particulate ma- [58] 17 36 terial from the outlet of the kiln or the heat input to e 0 care the kiln in response to differential pressure changes 7 between fluid, such as combustion-supporting fluid and process gas flowing into and from the burning or [56] References cued calcining zone, to result in a balance between particu- UNlTED STATES PATENTS late mass flow and heat input, and a substantially uni 2,625,386 1/1953 Leone 432/14 formly heat-treated product. 2,626,141 l/l953 Grossman .1 432/47 2,774,572 l2/l956 Goins 432 47 19 Clalms, 3 Drawing Flgures n I20 l DPT 22- l 1 1 76 l l 1 l l I i I l I 70 l 66 56 46 e 1 7 ""1660 6 60 65 a0 64 eat 36!:

VERTICAL KILN CONTROL This invention relates to vertical kilns. In another aspect, this invention relates to controlled heat treatment of particulate material within a vertical kiln. In still another aspect, this invention relates to a novel calcining process and apparatus.

Vertical heat treating vessels which are commonly known as vertical kilns, shaft kilns, shaft furnaces, or shaft generators, retorts or the like, depending upon the type of treatment and the material being treated, comprise process equipment commonly found in diverse kinds of industry. Such devices have been used for burning or calcining lime, coking coal, burning argillaceous and calcareous material in the production of cement clinker, burning magnesite, dolomite, retorting oil shale, and the like. Such kilns commonly include a vertical vessel having an elongated heating shaft therewithin, a means for uniformly feeding a particulate material into the elongated heating shaft, a lower discharge means for removing material from the lower outlet end of the kiln, and a means for introducing a stream of heat treating fluid through the particulate material. Commonly, the kilns include a means for introducing a combustible fluid such as a fuel-air mixture upwardly through the kiln which establishes a combustion or burning zone in the middle portion of the kiln. Other conventional kilns or retorts utilize a heat supply system which includes an external combustion system and means for directing the hot gases from the combustion systemto the burning zone of the kiln.

Problems have been encountered in maintaining a uniform downward movement of particulate material throughout the cross sectional extent of the vertical, shaft from the top or feed end of the kiln to the lower or outlet end of the kiln. As a result, discharge grates such as disclosed in U.S. Pat. No. 3,401,922 have been developed in an effort to solve this problem. Furthermore, problems have been encountered in uniformly heat treating the particulate material passing through the kiln, even thoughthe actual speed of the particulate mass passing through the kiln can be fairly accurately controlled by using discharge grate systems such as disclosed in the above-cited patent. In general, uniform heat treatment of a particulate mass passing through a vertical kiln has been difficult to effect because of the difficulty of maintaining the required heat input to the material passing through the burning zone.

It has generally been difficult to obtain a uniform quality ofreactive lime (CaO) from limestone (CaCO feed material with a vertical kiln. Very reactive lime (which is reactive to water for hydration) is basically in the rhombic crystalline form. Lime in a cubic crystalline form is nonreactive to water for hydration. Limestone generally has a rhombic crystalline structure and by careful control of the calcining process which includes burning of 'the limestone and removing CO therefrom, the rhombic crystalline structure of limestone can be retained in the lime. However, overburn in a kiln will alter the rhombic crystalline structure of the product, e.g., change it to cubic form, and there- 2% inch or more, it has been extremely difficult to obtain a uniform distribution of solids and treating fluids through the burning zone in the vertical kiln. As a result, the more reactive lines have been produced by the rotary kilns available in the art. The rotary kilns are extremely thermally inefficient but capable of producing a uniformly calcined and active product. Therefore, the lower grade limes are conventionally produced by the vertical kilns. Furthermore, lime produced by prior art vertical kilns can vary widely in product quality, because the gradation of the limestone fed to the kiln generally varies considerably.

Accordingly, one object of this invention is to provide a novel method and apparatus for controlling heat treatment of particulate material in a vertical kiln.

Another object of this invention is to provide a novel method and means for controlling the residence time of particulate material within the burning zone of a vertical kiln.

A further object of this invention is to provide a novel method and means for producing lime from limestone.

According to the invention, particulate material of a nonuniform gradation is heat treated in a burning zone of a vertical kiln by regulating the flow rate of particulate material from the outlet of the kiln and/or the heat input to the kiln in response to differential pressure changes between fluid such as combustion supporting fluid and process gas flowing into and from the burning zone, to thereby yield a balance between the particulate mass flow and heat input to produce a uniformly heat treated product.

According to one embodiment of the subject invention, particulate material of nonuniform gradation is heat treated in a burning zone of the vertical kiln having a substantially uniform heat input by continuously passing the material through the burning zone and controlling the flow of the material from the burning zone in response to differential pressure changes between fluid flowing into and from the burning zone to yield a substantially uniformly heat treated product.

According to a preferred embodiment of this invention, a method and apparatus are provided for heat treating particulate material of nonuniform gradation in a vertical kiln by supplying a heat input to the burning zone of the vertical kiln sufficient to treat a charge of average particles which have a predetermined mass residence time in the burning zone which is based upon a predetermined bulk density and a predetermined flow rate of particles through the burning zone, and thereafter passing the particulate materials into the burning zone and sensing a quality indicative of bulk density of the particulate materials passing through the burning zone, and comparing the indicated bulk density with said predetermined bulk density and thereafter controlling the flow of the particulate material through the burning zone relative to the compared bulk density to yield a mass residence time substantially equal to the predetermined mass residence time within said burning zone and a substantially uniform heat treatment of the particulate material.

According to a specifically preferred embodiment of said above-recited preferred embodiment, the difference in pressure of a fluid passed through the burning zone is measured at a point when the fluid enters the burning zone and a point when the fluid leaves the burning zone and the measured pressure differential is compared to a predetermined pressure differential of a fluid flowing through a mass of average particles having a predetermined mass residence time and a resultant heat treated quality, and, thereafter, the flow of particulate material through the burning zone is adjusted to yield a uniform product having said heat treated qual ity.

This invention can be more easily understood from a study of the drawings in which:

FIG. 1 is a schematic illustration of a vertical kiln equipped with a control mechanism of the subject invention;

FIG. 2 is a schematic diagram showing the control mechanism of FIG. 1 in greater detail; and

FIG. 3 is a partial view ofFlG. 2 showing the fourway valve in its second position.

Now referring to the drawings, and in particular to FlG. 1, vertical kiln l0,comprises a conventional vertical kiln having an internal hollow shaft within which particulate material is subjected to heat treatment. The inlet to kiln-l receives solid particulate material such as limestone, which is initially delivered from stone storage bin 12 by way of a conveyor 14 through rotary seal 16 into hopper 18. Level controller 180 operates discharge control mechanism 12a of storage bin 12.

Vertical'kiln is provided with a fuel and air delivery system adjacent its lower midportion for delivering a combustible mixture tothe burning zone of the kiln. The lower end 20 of the burning zone is schematically depicted by a broken line and the upper end 22 of the burning zone is schematically depicted by a broken line.

it is noted that a vertical kiln equipped with the control mechanism of the subject invention can utilize any heat supply system known in the art, e.g., external or internal combustion chambers. As illustrated in this embodiment, a gaseous fuel such as natural gas is delivered by gas inlet conduit 24 and feeds a manifold 26 from which gas'supply lines 28, and 31 depend. Flow controllers 28a, 30a and 31a operate flow control valves 2b 30b and 31b in gas supply lines 28, 30 and 31, respectively. Air under pressure is supplied from conduit 32 into air manifold 34 from which air lines 36, 38 and 40 emerge. Flow controllers 36a, 38a and 40a operate flow control valves 36b, 38b and 40b in air lines 36, 38 and 40, respectivlely.

As shown, gas supply line 28 communicates between air supply line 40 and fuel manifold 26, gas supply line 30 communicates between air supply line 38 and fuel manifold 26, and gas supply line 31 communicates between air supply line 36 and fuel manifold 26. Thus, the gas and air are mixed within air lines 38 and 40 and 36, if desired, prior to entrance into the kiln. The fluid from lines 36, 38 and 40 are passed into fluid distributor systems 42, 44 and 46, respectively, before being introduced as distributed streams as illustrated sche matically by flow arrows 42a, 44a, and 46a, respectively. Suitable such fluid distributor systems are disclosed in US. Pat. No. 3,432,348 or 3,589,611, which systems are herein incorporated by reference into this specification. The preferred such system is disclosed in U.S. Pat. No. 3,589,611.

The combustion supporting gas which is delivered by these fluid distributor systems, will provide fuel for the burning zone in the kiln, and allow proper heat treatment of the particulate material passing downwardly therethrough by gravitational force. The off-gases from the kiln are removed via stack 48.

The heat treated particulate material is passed from outlet 50 through rotary seal 52 onto product conveyor 54. "The flow of particulate material to outlet 50 is controlled by a grate control mechanism which operates in accordance with the subject invention. The grate control mechanism will be described in detail below. Grate 56 can be the linear grate for shaft kilns which is disclosed in US. Pat. No. 3,40l,922 which patent is herein incorporated by reference into this specifica tion. However, any other suitable grate known in the art can be used in the scope of this invention. Grate 56 basically comprises a series of spaced diverter plates 58, having retarder plates 60 positioned a spaced distance below the opening between adjacent diverter plates 58. Generally, the distance of the edge of each retarder plate 60 under each diverter plate 58 is determined by the angle of repose of the material on itself, which passes through the kiln. Pusher bars 61 are reciprocally mounted between diverter plates 58 and retarder plates 60. As illustrated in the embodiment shown in the drawing, half of the pusher bars 61 are interconnected by rods 62 and the other half are interconnected by rods 64. Rods 62 and 64 are connected by rods 65 and are controlled by the action of hydraulic cylinders 66 and 68, respectively. More specifically, rods 62, 64 and 65 move pusher bars 61 in reciprocal motion by the action of hydraulic cylinders 66 and 68, respectively. In essence, the controlled reciprocal movement of pusher bars 61 across retarder plates 60 controls the flow of material passing to the outlet 50 from openings between adjacent diverter plates 58.

The relative motion imparted to rods 62 and 64 by hydraulic cylinders 66 and 68, respectively, is regulated by grate speed controller 70. Grate speed controller 70 in turn is operatively connected to differential pressure transmitter 72. Pressure sensor 74 is positioned adjacent the lower end 20 of the burning zone within the kiln and is operatively connected to differential pressure transmitter 72 via line 76. Pressure sensor 78 is positioned at a point adjacent the upper end 22 of the burning zone within the kiln l0 and is operatively connected to differential pressure transmitter 72 via line 80.

A detailed view of a preferred control system used in the scope of the subject invention is schematically illustrated in FIG. 2. As shown, differential pressure transmitter 72 can comprise any suitable type differential pressure transmitter known in the art having two signal inputs and one signal output. A suitable such device is Honeywell Ap/P transmitter Model 292l2-010l. Thus, differential pressure transmitter 72 receives two pressure inputs from pressure sensors 74 and 78, compares these inputs, and transmits a signal representative of the difference of the two to control valve 82. It is noted that the combination of pressure sensor 78 and line and the combination of pressure sensor 74 and line 76 can each comprise a monometer tube. In this instance, it is desirable to pass a uniform flow of purge gas such as air through the monometer tubes. It is furthermore noted that pressure sensing means 74 and 78 can be positioned at any convenient spaced distance below and above, respectively, the burning zone in the kiln. However, it is generally preferred that pressure sensor 74 be positioned adjacent the lower end of the burning zone and that sensor 78 be positioned adjacent the upper end of the burning zone within kiln 10.

Now again referring to FIG. 2, the pistons 66a within hydraulic cylinders 66 are coupled to rods 62, and rods 62 carry pusher bars 61. Likewise, the pistons 68a within hydraulic cylinders 68 are operatively connected to rods 64, and rods 64 carry pusher bars 61. Rods 62 and 64 are interconnected by rods 65. Switch bars 84 and 86 depend from pusher bars 61 and function, as shown, to actuate contacts 88 and 90, respectively. Contacts 88 and 90 actuate a conventional valve control switch 91 which functions to alternately move four-way valve 102 between its first and second positions. 7

The hydraulic systemwhich is utilized to operate grate 56 includes a centrifugal pump 92 with an inlet conduit 94 operatively communicating between hydraulic fluid reservoir 96 and the inlet of pump 92. Conduit 98 communicates between the outlet of pump 92 and port 100 of four-way valve 102. Four-way valve 102 can be any conventional four-way valve unit known in the art. A suitable such four-way valve is a Racine Model No. OD4-DNHS-l02S. As shown in FIG. 2, four-way valve 102 is in its first position, which will thereby allow port 100 to communicate with port 104. Manifold conduit 106 operatively communicates with port 104. Conduits 108 and 110 operatively communicate between manifold conduit 106 and the front faces of the pistons 68a within hydraulic cylinders 68. Conduits 112 and 114 communicate between the rear faces of pistons 68a within hydraulic cylinders 68 and conduit 116. Conduit 116 operatively communicates with outlet manifold conduit 118. Outlet manifold conduit 118 communicates between hydraulic fluid reservoir 96 and conduit 120. Conduits 122 and 124 operatively communicate between conduit 120 and the rear faces of pistons 66a within hydraulic cylinders 66. Conduits 126 and 128 communicate between the front faces of pistons 66a within hydraulic cylinders 66 and conduit 130. Conduit 132 commmunicates between valve port 134 of four-way valve 102 and conduit 130. As shown, with four-way valve 102 in its first position, valve port '134 communicates with valve port 136 and valve port 136 operatively communicates with conduit 138, Flow control valve 140 is positioned within conduit 138 and conduit 142 comprises a by-pass loop communicating with conduit 138 on either side of flow control valve 140. Flow control valve 140 can be any conventional such valve known in the art. A suitable such valve is a constant volume, temperature and pressure compensated flow control valve such as a Racine Model F2AHS *-02* valve. Control valve 82 is positioned within conduit 142.

As previously set forth, control valve 82 is operated by signals from differential pressure transmitter 72 and can comprise any suitable control valve mechanism known in the art. For example, control valve 82 can comprise a Black, Sivalls, and Bryan Valve Operator type 70-13-10 and a Racine Model OF2-CHPW50H hydraulic valve. Conduit 138 communicates fromconduit 142 to hydraulic fluid reservoir 96. A filter 143 and a heat exchanger 144 are operatively positioned within conduit 138. In addition, by-pass conduit 146 is positioned around filter 143 with relief valve 148 positioned therein which will allow hydraulic fluid to by pass the filter when a predetermined hydraulic pressure is reached, e.g., in case of pressure surges or in in stances wherein the filter becomes clogged.

Now, referring to FIGS. 1-3, the operation of the control apparatus of the subject invention will be described in detail. Basically, the control apparatus as set forth in the drawing functions to control the heat treatment of particulate materials passing through vertical kiln 10 and assures a predetermined mass residence time within vertical kiln 10. Generally, particulate material which is fed to the vertical kiln 10 will vary in particle size and in gradation of the particles and accordingly, will vary in mass density.

Generally, particulate material such as previousl crushed and sized limestone delivered from storage bin 12 into hopper 18 on kiln 10 will have a mass density which varies from a fixed minimum to a fixed maximum. However, due to the fact that the particulate material is subjected to gravitation action not only within storage bin 12, but also within the interior of feeder hopper l8 and vertical kiln 10, the particle size graduation will not be constant. Therefore, in accordance with a preferred embodiment of this invention, the grate speed of controller 70 is calibrated by passing the crushed and sized particulate material, such as limestone, through the kiln having a relative constant heat input to the burning zone, and controlling the grate speed until a product having the desired degree of calcination is obtained, e.g., a product wherein the carbon dioxide content of the calcined limestone falls within the range of from Q weight percent of a control value, such as 3 or 3.5 wt. percent of the product. The calibration of the grate speed and differential pressure is basically linear in nature, and it is found that to obtain a product of cthe desired quality with the material having the nonuniform gradation that a substantially uniform mass residence time will pass through the burning zone. Thus, the term substantially uniform mass residence time" is herein meant to include a mass residence time which will yield a product having the predetermined or controlled degree of calcination when passed through the burning zone (a degree of calcination whichfalls within a desired range).

Generally, a relatively constant heat input is supplied to the burning zone in vertical kiln 10. This constant heat input is based upon an average or predetermined particle gradation and consequently, an average or predetermined mass density of particulate material which is passed through the heating zone to assure that proper heat treatment of the particulate material is effected without resulting in either overburn or underburn of the material as described above. The material of the predetermined mass density will effect a predetermined pressure drop of fluid passing through the burning zone, e.g., the air and fuel mixture and process gases released by calcination and gaseous combustion products of the mixture which is passed upwardly through kiln 10 from fluid distributor systems 42, 44 and 46. Thus, valve is set at a predetermined opening and functions in combination with control valve 82 to control the amount of hydraulic fluid passing through conduit 138 and thereby controls the speed of grate 56. When material enters the burning zone which has either higher or lower porosity than the standard mate rial of predetermined particle gradation (i.e., has a higher or lower mass density) differential pressure of the fluid passing upwardly through the burning zone will accordingly be altered and the differential pressure input to control valve 82 will adjustcontrol valve 82 which in turn adjusts flow through conduit 138 and alters the speed of gate 56. Thus, valve 82 is calibrated to control the speed of grate 56 in response to variations of differential pressure outputs from differential pressure transmitter 72. In essence, if material enters the burning zone of the kiln which has a greater mass density and thereby lower porosity than the predetermined or average mass density, differential pressure transmitter 72 will indicate an increase in differential pressure between the lower and upper portion of the burning zone. This will effect a closure of valve 82 and a slowing down of grate 56 which will allow a longer burning time for the higher mass density material entering the zone such that the material will have an equivalent mass residence time to that of the material with the predetermined mass density and thereby yield a substantially uniform calcined product. Alternately, if the material entering the burning zone has a lower mass density and therefore a greater porosity than the material of predetermined particle size, then the differential pressure between the upper and lower portions of the burning zone will be less than that which corresponds to the material ofpredetermined particle size and the differential pressure transmitter will effect an opening of control valve 82, and thereby allow grate 56 to operate at a faster rate so that the resulting mass residence time of the higher porosity lower mass density material is equivalent to that of the material of predetermined particle size and again yield a substantially unifom calcined product.

Thus, referring to FIGS. 2 and 3, the operation of the preferred embodiment of the subject invention will be discussed in detail. Initially, valve 140 is adjusted so that the flow of fluid therethrough in combination with the flow of fluid through valve 82 will result in a grate speed which is sufficient to yield a predetermined mass residence time of particulate material of predetermined particle size passing through the burning zone of kiln 10. Now, with four-way valve 102 in its first position as illustrated in FIG. 2, pump 92 is run at a constant speed and constantly withdraws hydraulic fluid from reservoir 96 via conduit 94 and passes the hydraulic fluid to port 100 of four-way valve 102 via conduit 98. The fluid passes through four-way valve 102, port 104, and into conduit 106 and conduits 108 and 110, thereby passing fluid into the front portion of hydraulic cylinders 68 and against the front faces of pistons 68a therewithin. This causes a retraction of pusher rods 64 and a movement of retarder plates 60. Since pusher rods 64 are interconnected to rods 62 by rods 65, this also causes an extension of rods 62 and results in the front faces of piston 66a of hydraulic cylinder 66 forcing fluid to conduit 130 via conduits 126 and 128. Furthermore, the retraction of pistons 680 causes fluid to pass through conduits 112 and 114 to outlet manifold conduit 118 and into conduits 120, 122, 124 and also into reservoir 96. Fluid from conduit 130 passes to conduit 132 into valve port 134 through four-way valve 102 to valve port 136 and into conduit 138.

The fluid passes through conduit 138, through conduit 142, control valve 82, valve 140, through filter 143, heat exchanger 144, wherein the fluid is cooled and back to reservoir 96. This action continues until switch bar 86 touches contact 90. When contact 90 is actuated, valve control switch 91 moves four-way valve 102 to its second position as illustrated by the partial view in FIG. 3. In this instance, the pump output flowing through conduit 98 to valve port 100 passes directly to valve port 134 into conduits 132, 130 and 126 and 128 to the front face of pistons 66a and hydraulic cylinders 66. This action causes pistons 66a to retract, and rods 62, pusher bars 61 and rods 64 to be moved toward hydraulic cylinders 66. This action causes fluid against the rear face of piston 66a to pass through conduits 122, 124 and to conduits 120, and 118, 114, 112 to the rear faces of pistons 68a of hydraulic cylinders 68. This in turn will force fluid which is in contact with the front faces of pistons 68a of hydraulic cylinders 68 into conduits 108, 110, 106 into valve port 104 of fourway valve 102.

The fluid passes through four-way valve 102 to valve port 136 into conduit 138 and again through conduit 142, control valve 82, filter 143, heat exchanger 144, back to the reservoir 96. This action continues until switch bar 84 actuates contact 88 which in turn actuates valve control switch 91 which moves four-way valve 102 again to its first position, at which time the sequence is repeated. As can be seen, changes from the differential pressure transmitter 72 alter the opening of valve 82, and thereby controls the speed of grate mechanism 56. More specifically, when four-way valve 102 is in its first position and fluid from centrifugal pump 92 is being pumped against the front faces of pistons 680 within hydraulic cylinders 68 and thereby causing them to retract within the hydraulic cylinders 68, rods 62 are extending from hydraulic cylinders 66 and thereby, the front faces of pistons 66a within hydraulic cylinders 66 are forcing fluid through the outlet flow path toward reservoir 96 which includes a passage through valves 140 and control valve 82. Thus, the back pressure imparted on the system by an oepning or closing of control valve 82 will affect the speed at which the fluid from the centrifugal pump 92 will move pistons 68a and 66a.

The following examples are given to better facilitate the understanding of this invention and are not intended to limit the scope thereof:

EXAMPLE 1 An apparatus such as illustrated in FIGS. l-3 was utilized to calcine limestone. The crushed and sized limestone which was calcined generally had a particle size ranging from about 3 4 inch to about 2% inches. The gradation of the limestone varied substantially but it generally had a mass density in the range of from about 76 to about 86 pounds per cubic foot. Due to the tendency of the smaller particles to gravitate downwardly within storage bin 12 and kiln 10, the gradation of the limestone passing through the burning zone of kiln 10 will vary considerably with time. As an example, limestone which had a mass density ranging from about 76 to about 86 pounds per cubic foot and which was delivered from storage bin 12 over a period of 8 days was measured for particle size distribution 2 or 3 times a day and the results are shown in Table l below.

Table I STONE GRADATION Retained on Screed Table l-Continued STONE GRADATION Retained on Screed Day Time 1%" 1" 5 1" A" Ave. 31.3 36.0 19.6 10.1 3.0 Deviation 16.6 10.3 9.8 8.6 4.2

As can be seen, the gradation of the limestone delivered from bin 12 varied tremendously with time, even though the mass density only ranged from 76 to 86 pounds per cubic foot.

Kiln was initially set to operate with natural gas entering conduit 24 and air entering conduit 32 to esto close valve 31a and allow no gas to pass through conduit 31); a total of about 1,305 standard cubic feet per minute of a rich gas-air mixture which consisted of a ratio of about 4.9 standard cubic feet of air to about 3 standard cubic feet of fuel delivered through fluid distributor system 44; and about 2,755 standard cubic feet per minute of a lean fuel-air mixture was passed through fluid distributor system 46 and consisted of a ratio of about 5.8 standard cubic feet of air to about 1 standard cubic foot of fuel.

Control valve 82 was calibrated such that the grate speed of grate 56 varied in response to a change in the density of the limestone passing through the burning zone between differential pressure sensors 74 and 78 as determined by changes in the differential pressure of fluid passing therethrough. The grate speed was correlated with each differential pressure increment within this range to yield a product which contained about 3 percent i 1 wt. percent of carbon dioxide and thereby yield a substantially uniform mass flow rate through the burning zone. It was specifically found that for a mass density range'of from about 76 to about 86 pounds per cubic foot, a corresponding differential pressure range of about 7 inches of water would result. This differential pressure range was used to control the valve 82. In essence the average" grate speed setting corresponded to a differential pressure of the fluid passing through the burning zone which would indicate that the mass density of the material therein was about 81 pounds per cubic foot; the fastest grate speed setting corresponded to a'differential pressure which indicated the material had a mass density of about 76 pounds per cubic foot; and the slowest grate speed setting corresponded to a differential pressure which indicated that the mass density of the material passing through the burning zone was about 86 pounds per cubic foot.

After the instruments were calibrated, the limestone having the above-described gradation and having a density variation of between about 76 and about 86 pounds per cubic flow was passed to kiln 10 operating as set forth above, at a feed rate of about 26.5 tons per hour. A 24 hour run showing the variance in pressure as measured by differential pressure controller 72 is set forth in Table 11 below:

The differential pressure varied from a low of 16.8 inches of water to a high of 23.7 inches of water with corresponding grate speeds ranging from 62.4 complete strokes of pusher bars 61 (a complete reciprocal motion across kiln 10) to 64.9 complete strokes. The average carbon dioxide content of the calcined limestone removed from outlet 50 (as determined by ASTM 25-29, Ascarte method) was about 3 weight percent, and it ranged from a low of 2.3 wt. percent to a high of 3.3 wt. percent.

As can be seen, the control mechanism which was utilized in accordance with the subject invention resulted in a substantial uniform product quality.

EXAMPLE 2 Particulate limestone such as described in Example 1 and having a mass density which ranges from about 76 to about 86 pounds per cubic foot was delivered to the internal shaft of vertical kiln 10 and subjected to heat treatment within the burning zone thereof operating at a temperature between 1,500" and 2,800 F. This was accomplished by delivering a total of about 7,251 standard cubic feet per minute of air through fluid distributor 42; a total of about 1,305 standard cubic feet per minute of a richgas-air mixture which consists of a ratio of about 4.9 standard cubic feet of air to about 3 standard cubic feet of fuel delivered through fluid distributor system 44; and about 2,755 standard cubic feet per minute of a lean fuel-air mixture through fluid distributor system 46 which consisted of a ratio of about 5.8 standard cubic feet of air to about 1 standard cubic foot of fuel. The grate control mechanism had been initially calibrated in a manner set forth within Example 1 to pass the particulate limestone through the burning zone and yield a calcined product having about 3.5 i 1.5 weight percent CO therein.

The kiln was allowed to automatically operate solely in response to the differential pressure control mechanism' of the subject invention for 4 days at an average feed rate of from 22 to 24 tons of limestone per hour. The exact tons per each day are set forth in Table III below. The calcined product from outlet 50 was peri-. odically analyzed for carbon dioxide in accordance with ASTM 25-69, Ascarite method, and the average weight percent CO in the product was noted. In addition, samples were periodically taken from the calcined product and subjected to a standard water reactivity test set forth by the American Waterworks Association and designated AWWA 202-65. During this test the samples were contacted with water and the temperature rise over 1 minute, 3 minutes, and 6 minutes was pressure transmitter 72 can actuate the fuel-air control system to thereby supply a predetermined heat increase to the heat treating zone to thereby compensate for the greater mass density material. Likewise, when material of lower mass density than the average is passed to the heat treating zone, the fuel-air system can be proportionally cut back.

While this invention has been described in relation to its preferred embodiments, it is to be understood that various modifications thereof will now be apparent to one skilled in the art on reading this specification and it is intended to cover such modifications as fall within the scope of the appended claims.

recorded. These data are set forth in Table 11] below: I claim: Table III Reactivity Hrs. of Tons Wt 7c CO ("C rise) Operaof in Product (average) tion Stone l 3 Fed Day Average Maximum Minimum Min Mins. Mins.

l 24 576 3.6 4.7 2.9 22 24 23 2 24 S56 3.6 5.4 2.3 21 24 23 3 24 547 i 3.6 4.3 3.1 23 26 25 4 21 473 3.7 5.0 2.9 24 27 26 As can be seen, the average C content in the product is relatively constant. This indicates a uniform degree of calcination of the product. Furthermore, the reactivity of the lime produced over the 4-day period as indicated by the reactivity test also indicates a uniformly active lime product. As a comparison, when the same kiln is operated without the control mechanism of the subject invention wherein the rate of the grate 56 was controlled by the weight of stone passed into the vertical kiln, and the kiln is run under the same conditions of temperature and using the same limestone feed which ranges in mass density of from about 76 to about 86 poundsper cubic foot, the average CO content will range from less than.3, to more than 8. The reactivity of the product for the l. 3, and 6 minute tests varies over a range of about C. Thus, it is apparent that by operating with the differential pressure control mechanism of the subject invention, a more uniformly active lime product can be obtained by the continuous operation of kiln 10.

It is to be noted that the subject invention can be utilized for control of any vertical kiln, furnace, retort, or the like, which is conventionally utilized to heat treat any particulate material. For example, the subject invention can be used not only for the calcining of lime but for the coking of coal, for burning argillaceous and calcareous material in the production of cement clinker, burning magnacite, dolomite, but also for retorting oil shale. Furthermore, the differential pressure control system of the subject invention can be utilized to not' l. A process for heat treating particulate material of nonuniform gradation in a vertical kiln having a particulate material inlet at the upper end thereof and a particulate material outlet at the lower end thereof comprising:

a. supplying a heat input to the burning zone of said kiln sufficient to treat a charge of standard particles having a predetermined mass residence time which is based upon a predetermined bulk density and a predetermined flow rate of particles therethrough;

b. delivering said particulate material to said inlet of said vertical kiln at a sufficient rate to maintain said kiln filled with particulate material from said outlet through said burning zone, passing said particulate materials into the burning zone of said kiln, and continuously removing heat treated particulate material from said outlet;

c. sensing a quality indicative of the bulk density of said particulate materials passing through said burning zone; and

d. controlling the rate of said particulate material passing through said burning zone in step (b) relative to a comparison of said quality with a quality indicative of said predetermined bulk density to yield a mass residence time equivalent to said predetermined mass residence time within said burning zone and substantially uniformly heat treated particulate materials.

2. The process of claim 1 wherein said flow of particulate material is controlled through said burning zone by controlling the flow of said particulate material through the outlet of said vertical kiln.

3. The process of claim 1 wherein said particulate material is limestone.

4. A process of calcining particulate material of nonuniform gradation in a vertical kiln comprising:

a. supplying a heat input to the burning zone of said vertical kiln by combusting a fuel-air mixture which is passed upwardly through said burning zone;

b. maintaining the heat input to said burning zone at a constant level which is sufficient to calcine a charge of particles having a predetermined mass residence time which is based upon a predetermined bulk density and a predetermined flow rate of particles passing therethrough;

c. passing said particulate materials of nonuniform gradationinto the burning zone of said kiln;

d. measuring the pressure of fluid in said vertical kiln passing upwardly to said burning zone, and the pressure of fluid passing upwardly through said vertical kiln from said burning zone to obtain a measured differential pressure across said burning zone;

d. comparing said measured pressure differential with a predetermined pressure differential which corresponds to the pressure differential of said fluid passing upwardly through said burning zone containing said charge of standard particles having a predetermined mass residence time; and

f. regulating the flow of said particulate material through said burning zone relative to the compared differential pressure to yield a mass residence time of said particulate material in said burning zone which is substantially equal to said predetermined mass residence time within said burning zone.

5. The process of claim 4 wherein said particulate material is limestone.

6. A process for calcining particulate material of nonuniform gradation in a vertical kiln comprising:

a. delivering said particulate material to the inlet of a vertical kiln so that a uniform level of particulate material is maintained in said kiln;

b. supplying a constant heat input to the burning zone of said kiln by burning a combustible fluid mixture which is passed upwardly through said burning zone, said constant heat input being sufficient to calcine a charge of standard particles to a predetermined degree, which standard particles have a predetermined mass residence time within said burning zone which is based upon a predetermined bulk density of said standard particles and a predetermined flow rate of said standard particles therethrough;

. measuring the difference in pressure between said fluid passing to said burning zone and said fluid passing from said burning zone; and

d. regulating the flow of said particulate material through said burning zone relative to changes in said differential pressure to yield a product calcined to said predetermined degree.

7. The process of claim 6 wherein said particulate material is limestone.

8. The process of claim 6 wherein said particulate material is delivered to the inlet of said vertical kiln at a sufficient rate to maintain said kiln filled with particulate material from the outlet through said burning zone.

9. The process of claim 8 wherein the flow of said particulate material is regulated through said burning zone by controlling the flow of said particulate material from said outlet.

10. In a process for calcining particulate material of nonuniform gradation in a vertical kiln wherein the particulate material is passed to the inlet of the vertical kiln and is allowed to gravitate through a burning zone in the kiln and is thereafter removed from the outlet at the lower end of said kiln, the improvement comprismg:

regulating the flow of particulate material from said outlet to thereby yield a substantially uniform mass residence time of said particulate material within said burning zone.

11. The process of claim 10 wherein said particulate material is limestone.

l2. ln a vertical kiln having means to feed particulate material to its upper inlet end thereof; means to pass a fuel-air mixture upwardly through said vertical kiln and establish a burning zone within the midportion thereof; and an outlet grate means to remove heat-treated particulate material from the lower outlet end thereof at a controlled rate; the improvement comprising:

means for measuring the pressure of the fluid flowing upwardly to a particulate mass in said burning zone, and means to measure the pressure of the fluid flowing upwardly from the particulate mass in the burning zone, and means for obtaining a differential pressure therebetween, and means for controlling the rate of said outlet grate means in response to fluctuations in said differential pressure. 7

13. A vertical kiln for heat treating particulate material comprising:

a. an elongated heating chamber having an upper inlet end, a lower outlet end, and a burning zone therebetween;

b. means for supplying fluid to the interior of said elongated vertical heating chamber to thereby pass upwardly through said burning zone;

c. grate means positioned in the outlet of said kiln for removing heat treated particulate material therefrom at a controlled rate;

d. means to measure the pressure of said fluid flowing to a particulate mass in said burning zone, means to measure the pressure of said fluid flowing from a particulate mass in said burning zone, and means for obtaining a differential pressure therebetween; and

e. means operatively connected to said means for obtaining a differential pressure for, regulating the rate of said outlet grate means in response to fluc' tuations in said differential pressure.

14. The vertical kiln of claim 13 wherein said means for supplying fluid comprises a means for supplying a fuel-air combustion supporting fluid to the interior of said elongated heating chamber.

15. The vertical kiln of claim 14 further comprising means to supply particulate material to the inlet of said elongated vertical heating chamber to maintain a constant level of particulate material within said chamber.

16. The vertical kiln of claim 15 wherein said grate means comprises a series of spaced-apart generally parallel positioned deflector plates extending across the outlet of said elongated heating chamber and providing at least one opening therebetween in said outlet, retainer plate means positioned below and spaced from each said opening and having its edges extending under the edge of each deflector plate to prevent a space therebetween along vertical lines; pusher means reciprocally mounted in the space between said deflector plates and said retainer plates, and means for reciprocally moving said pusher means across each said retainer plate.

17. The vertical kiln of claim 16 wherein said means for reciprocally moving said pusher means comprises first and second opposed hydraulic cylinder means each comprising internal piston means having a front and back face and a piston rod attached to each front face and extending outwardly therefrom and positioned in an opposed manner through opposite walls of said elongated vertical heating chamber and being operatively connected to said pusher means.

18. The vertical kiln of claim 17 wherein said means to control the outlet rate of said grate in response to fluctuations in said differential pressure comprises a hydraulic fluid flow control means operatively connected to said first and second opposed hydraulic cylinder means.

19. The vertical kiln of claim 18 wherein said hydraulic fluid flow control means comprises:

a. hydraulic pump means for delivering hydraulic fluid and hydraulic reservoir means for delivering hydraulic fluid to said hydraulic pump means;

b. a four-way valve means having first and second ports which alternately communicate with third and fourth valve ports, such that when in its first position, the first valve port communicates with the fourth valve port and the second valve port communicates with the third valve port, and when in its second position, the first valve port communicates with the fourth valve port and the second valve port communicates with the third valve port;

0. first conduit means communicating with the outlet of said hydraulic pump and said first valve port of said fourway valve means;

01. second conduit means communicating between said fourth valve port of said four-way valve means and the space adjacent said front face of said piston means within said first hydraulic cylinder means;

e. third conduit means communicating between said third valve port and the space adjacent said front face of said piston means within said second hydraulic cylinder means;

f. fourth conduit means communicating between the space adjacent said rear face of said. piston means within said second hydraulic cylinder and the space adjacent said rear face of said piston means within said first hydraulic cylinder means; and

g. fifth conduit means communicating between said second valve port of said four-way valve means and said hydraulic reservoir means, and having a flow control valve means positioned therewithin which regulates the flow therethrough in response to said differential pressure. 

1. A process for heat treating particulate material of nonuniform gradation in a vertical kiln having a particulate material inlet at the upper end thereof and a particulate material outlet at the lower end thereof comprising: a. supplying a heat input to the burning zone of said kiln sufficient to treat a charge of standard particles having a predetermined mass residence time which is based upon a predetermined bulk density and a predetermined flow rate of particles therethrough; b. delivering said particulate material to said inlet of said vertical kiln at a sufficient rate to maintain said kiln filled with particulate material from said outlet through said burning zone, passing said particulate materials into the burning zone of said kiln, and continuously removing heat treated particulate material from said outlet; c. sensing a quality indicative of the bulk density of said particulate materials passing through said burning zone; and d. controlling the rate of said particulate material passing through said burning zone in step (b) relative to a comparison of said quality with a quality indicative of said predetermined bulk density to yield a mass residence time equivalent to said predetermined mass residence time within said burning zone and substantially uniformly heat treated particulate materials.
 2. The process of claim 1 wherein said flow of particulate material is controlled through said burning zone by controlling the flow of said particulate material through the outlet of said vertical kiln.
 3. The process of claim 1 wherein said particulate material is limestone.
 4. A process of calcining particulate material of nonuniform gradation in a vertical kiln comprising: a. supplying a heat input to the burning zone of said vertical kiln by combusting a fuel-air mixture which is passed upwardly through said burning zone; b. maintaining the heat input to said burning zone at a constant level which is sufficient to calcine a charge of particles having a predetermined mass residence time which is based upon a predetermined bulk density and a predetermined flow rate of particles passing therethrough; c. passing said particulate materials of nonuniform gradation into the burning zone of said kiln; d. measuring the pressure of fluid in said vertical kiln passing upwardly to said burning zone, and the pressure of fluid passing upwardly through said vertical kiln from said burning zone to obtain a measured differential pressure across said burning zone; d. comparing said measured pressure differential with a predetermined pressure differential which corresponds to the pressure differential of said fluid passing upwardly through said burning zone containing said charge of standard particles having a predetermined mass residence time; and f. regulating the flow of said particulate material through said burning zone relative to the compared differential pressure to yield a mass residence time of said particulate material in said burning zone which is substantially equal to said predetermined mass residence time within said burning zone.
 5. The process of claim 4 wherein said particulate material is limestone.
 6. A process for calcining particulate material of nonuniform gradation in a vertical kiln comprising: a. delivering said particulate material to the inlet of a vertical kiln so that a uniform level of particulate material is maintained in said kiln; b. supplying a constant heat input to the burning zone of said kiln by burning a combustible fluid mixture which is passed upwardly through said burning zone, said constant heat input being suFficient to calcine a charge of standard particles to a predetermined degree, which standard particles have a predetermined mass residence time within said burning zone which is based upon a predetermined bulk density of said standard particles and a predetermined flow rate of said standard particles therethrough; c. measuring the difference in pressure between said fluid passing to said burning zone and said fluid passing from said burning zone; and d. regulating the flow of said particulate material through said burning zone relative to changes in said differential pressure to yield a product calcined to said predetermined degree.
 7. The process of claim 6 wherein said particulate material is limestone.
 8. The process of claim 6 wherein said particulate material is delivered to the inlet of said vertical kiln at a sufficient rate to maintain said kiln filled with particulate material from the outlet through said burning zone.
 9. The process of claim 8 wherein the flow of said particulate material is regulated through said burning zone by controlling the flow of said particulate material from said outlet.
 10. In a process for calcining particulate material of nonuniform gradation in a vertical kiln wherein the particulate material is passed to the inlet of the vertical kiln and is allowed to gravitate through a burning zone in the kiln and is thereafter removed from the outlet at the lower end of said kiln, the improvement comprising: regulating the flow of particulate material from said outlet to thereby yield a substantially uniform mass residence time of said particulate material within said burning zone.
 11. The process of claim 10 wherein said particulate material is limestone.
 12. In a vertical kiln having means to feed particulate material to its upper inlet end thereof; means to pass a fuel-air mixture upwardly through said vertical kiln and establish a burning zone within the midportion thereof; and an outlet grate means to remove heat-treated particulate material from the lower outlet end thereof at a controlled rate; the improvement comprising: means for measuring the pressure of the fluid flowing upwardly to a particulate mass in said burning zone, and means to measure the pressure of the fluid flowing upwardly from the particulate mass in the burning zone, and means for obtaining a differential pressure therebetween, and means for controlling the rate of said outlet grate means in response to fluctuations in said differential pressure.
 13. A vertical kiln for heat treating particulate material comprising: a. an elongated heating chamber having an upper inlet end, a lower outlet end, and a burning zone therebetween; b. means for supplying fluid to the interior of said elongated vertical heating chamber to thereby pass upwardly through said burning zone; c. grate means positioned in the outlet of said kiln for removing heat treated particulate material therefrom at a controlled rate; d. means to measure the pressure of said fluid flowing to a particulate mass in said burning zone, means to measure the pressure of said fluid flowing from a particulate mass in said burning zone, and means for obtaining a differential pressure therebetween; and e. means operatively connected to said means for obtaining a differential pressure for regulating the rate of said outlet grate means in response to fluctuations in said differential pressure.
 14. The vertical kiln of claim 13 wherein said means for supplying fluid comprises a means for supplying a fuel-air combustion supporting fluid to the interior of said elongated heating chamber.
 15. The vertical kiln of claim 14 further comprising means to supply particulate material to the inlet of said elongated vertical heating chamber to maintain a constant level of particulate material within said chamber.
 16. The vertical kiln of claim 15 wherein said grate means comprises a series of spaced-apart generally parallel positioned deflector plates extendiNg across the outlet of said elongated heating chamber and providing at least one opening therebetween in said outlet, retainer plate means positioned below and spaced from each said opening and having its edges extending under the edge of each deflector plate to prevent a space therebetween along vertical lines; pusher means reciprocally mounted in the space between said deflector plates and said retainer plates, and means for reciprocally moving said pusher means across each said retainer plate.
 17. The vertical kiln of claim 16 wherein said means for reciprocally moving said pusher means comprises first and second opposed hydraulic cylinder means each comprising internal piston means having a front and back face and a piston rod attached to each front face and extending outwardly therefrom and positioned in an opposed manner through opposite walls of said elongated vertical heating chamber and being operatively connected to said pusher means.
 18. The vertical kiln of claim 17 wherein said means to control the outlet rate of said grate in response to fluctuations in said differential pressure comprises a hydraulic fluid flow control means operatively connected to said first and second opposed hydraulic cylinder means.
 19. The vertical kiln of claim 18 wherein said hydraulic fluid flow control means comprises: a. hydraulic pump means for delivering hydraulic fluid and hydraulic reservoir means for delivering hydraulic fluid to said hydraulic pump means; b. a four-way valve means having first and second ports which alternately communicate with third and fourth valve ports, such that when in its first position, the first valve port communicates with the fourth valve port and the second valve port communicates with the third valve port, and when in its second position, the first valve port communicates with the fourth valve port and the second valve port communicates with the third valve port; c. first conduit means communicating with the outlet of said hydraulic pump and said first valve port of said four-way valve means; d. second conduit means communicating between said fourth valve port of said four-way valve means and the space adjacent said front face of said piston means within said first hydraulic cylinder means; e. third conduit means communicating between said third valve port and the space adjacent said front face of said piston means within said second hydraulic cylinder means; f. fourth conduit means communicating between the space adjacent said rear face of said piston means within said second hydraulic cylinder and the space adjacent said rear face of said piston means within said first hydraulic cylinder means; and g. fifth conduit means communicating between said second valve port of said four-way valve means and said hydraulic reservoir means, and having a flow control valve means positioned therewithin which regulates the flow therethrough in response to said differential pressure. 